1. Field of the Invention
The present invention relates to a secondary battery that includes a spirally wound electrode body in which a cathode and an anode are layered with an electrolyte in between and spirally wound.
2. Description of the Related Art
In recent years, many portable electronic devices such as a combination camera (Videotape Recorder), a mobile phone, and a notebook personal computer have been introduced, and downsizing and weight saving of such devices have been made. Research and development for improving the energy density of the battery used as a portable power source for such electronic devices, in particular the secondary battery as a key device has been actively promoted. Specially, a nonaqueous electrolyte secondary battery (for example, lithium ion secondary battery) provides the higher energy density compared to a lead battery or a nickel cadmium battery as an existing aqueous electrolytic solution secondary battery. Therefore, improvement thereof has been considered in respective fields.
As an anode active material used for the lithium ion secondary battery, carbon materials such as non-graphitizable carbon and graphite, which show the relatively high capacity and the favorable cycle characteristics, have been widely used. However, taking account of the demand for the high capacity in these years, it is a task to obtain the higher capacity of the carbon material.
From such a background, a technique for attaining the high capacity with the use of the carbon material by selecting a carbonized raw material and preparation conditions has been developed (for example, refer to Japanese Unexamined Patent Application Publication No. 8-315825). However, in the case that such a carbon material is used, the anode discharge potential to lithium (Li) is from 0.8 V to 1.0 V, and the battery discharge voltage when forming the battery becomes low, and therefore significant improvement is not expected in terms of the battery energy density. Further, there are disadvantages that hysteresis is large in the shape of a charge and discharge curve, and energy efficiency in each charge and discharge cycle is low.
Meanwhile, as a high-capacity anode exceeding the carbon material, researches on alloy materials have been also promoted. Such alloy materials apply the fact that a certain metal is electrochemically alloyed with lithium, and the alloy is reversibly generated and decomposed. Further, it is considered that, as a method to improve the cycle characteristics, tin and silicon (Si) are alloyed to suppress expansion of tin and silicon. For example, it is proposed that a transition metal such as iron and tin are alloyed (refer to Japanese Unexamined Patent Application Publication Nos. 2004-22306, 2004-63400, and 2005-78999, “Journal of The Electrochemical Society,” 1999, Vol. 146, p. 405, p. 414, and p. 423). In addition, Mg2Si or the like has been suggested (refer to “Journal of The Electrochemical Society,” 1999, Vol. 146, p. 4401).